The atmospheric turbulence limits the performance of high resolution instruments. Adaptive Optics (AO) is a real time technique which compensates for the turbulent phase using a Deformable Mirror (DM) located in the instrument pupil. When a significant amount of turbulence is far away from the pupil, the AO performance is however limited by anisoplanatism and scintillation effects. For astronomical applications anisoplanatism effect is dominant, and can be corrected with several DMs conjugated with different turbulent layers ahead of the pupil. Recent studies have shown that such a concept of Multi-Conjugate Adaptive Optics (MCAO) can provide high resolution images in a large field of view. The goal of this study is to show that, in more severe turbulence conditions encountered in endoatmospheric applications, MCAO can also correct for scintillation effects, whereas classical AO is ineffective in this case. It appears that outside the weak perturbation turbulence domain, the perturbations of the nearest turbulent layers have to be corrected first in order to counteract the turbulence in the whole volume. This implies the use of relay optics for each turbulent layer in practical MCAO system designs. A simplified MCAO configuration is considered to study the number of DMs required to obtain a significant reduction of the scintillation.
High resolution imaging with adaptive optics and numerical image restoration processing is now an operational reality for ground-based telescopes. It provides a real-time compensation for turbulence degraded images. However, these correction techniques are only effective in a limited field of view due to anisoplanatism. For endoatmospheric applications, strong intensity fluctuations may also limit the degree of correction. In this paper, we estimate the isoplanatic domain as a function of the degree of correction of high resolution imaging systems and the transition between weak and strong intensity fluctuation regimes. We show that the saturation onset of scintillation corresponds to an isoplanatic field which becomes smaller than the resolution of adaptive optics systems.
Adaptive optics is now an operational reality which provides a real time compensation for turbulence degraded images. The degree of correction of an adaptive optics system is fundamentally limited by the flux density received from the object or from a guide source, and by atmospheric turbulence strength and speed. The ultimate performance which can be expected for ideal systems with an optimization of the key parameters are estimated. Two types of systems are routinely operated to date: a Hartmann-Shack wave-front sensor with a discrete actuator deformable mirror and a curvature wave- front sensor with a bimorph mirror. They are compared from physical and technological standpoints.
Adaptive optics and related techniques are now routinely used in the field of astronomical observations to compensate images for turbulence induced degradations and to retrieve the telescope diffraction limited resolution. However, for ground-to-ground applications, the effects of turbulence on both passive and active systems are more severe. The environmental constraints are also more important. Nevertheless, adaptive optics issues exist. The purpose of this paper is to review the evolutions of turbulence limitations according to the observing scenario. The turbulence anisoplanatism effects are emphasized. Furthermore, potential fields of application of adaptive optics and related techniques are presented.
From the local wavefront slope measurement given by a Shack Hartmann wavefront sensor (SH), it is possible to evaluate the decentering aberrations introduced by optical system misalignments. These slopes can be expressed as a function of the centered system aberrations and of the optical axis displacements in the image and pupil planes. After calibration of the wavefront sensor, the system alignment can be, for small aberrations, automatically controlled. The validity of the concept was proved on a representative observation system.
Wavefront sensing is a very powerful technique whose capability in the field of diffraction- limited imaging through turbulence has been demonstrated. The ultimate performance of a Hartmann-Shack wavefront sensor is analyzed and used to define a detector choice strategy.
Atmospheric turbulence is a major limitation to optical high resolution imaging systems observing through the lower atmosphere. In most conditions, it limits the angular resolution to about one arcsecond, which is the resolution of a small 10 cm telescope operating in the visible. From a theoretical standpoint, its effects on beam propagation can be rather well predicted with the knowledge of atmospheric conditions and the assumption of the Kolmogorov turbulence law. Then, the practical problem turns out to be the characterization of propagation conditions and the compensation of turbulence effects.
A bimorph mirror seems to be a low voltage, large strokes device which can be used
as a correction mirror in an adaptive optics system for infrared applications.
A few theoretical results are recalled and have been used to develop a numerical
method to solve the displacements of a bimorph mirror supplied by a distribution of
voltages. An example is given which involves seven electrodes ; comparisons with
theoretical and other numerical results are achieved.
Same paper has been presented during the SPIE's 90 Symposium on Astronomical
Telescopes and Instrumentation for the 21st Century in February 1990.
A bimorph mirror seems to be low voltage, large strokes device which can be used as a correction mirror in an adaptive optics system for infrared applications. A few theoretical results are recalled and have been used to develop a numerical method to solve the displacements of a bimorph mirror supplied by a distribution of voltages. An example is given which involves seven electrodes; comparisons with theoretical and other numerical results are achieved.